1. Field of the Invention
The present invention relates generally to graphics systems and in particular, discloses a method and apparatus for rendering object based graphics for real-time display of an image without the use of a frame store.
2. Description of the Related Art
The background art will now be described with reference to the following drawings in which:
FIG. 1 illustrates the outline of an object formed from a number of lines
FIG. 2 illustrates the associated y-bucket list of edge information of the prior art
FIG. 3 illustrates the current line edge list of the prior art as it appears while processing the scan line
Referring now to FIG. 1, FIG. 2 and FIG. 3, there is illustrated a traditional scan conversion method for an image made up of a list of objects, whereby each object's boundary edges are approximated by a polygon 101 and the polygon 101. Polygon 101 is made up of a number of edges 102, 103, 104, 105, 106 and 107. These edges are normally stored as a list of edges 108, where each edge is sorted by a maximum y co-ordinate value, and the list of edges 108 makes provision for at least one edge for each line.
Each edge list entry 109 includes information needed to generate scan line intersections, including x co-ordinate values, the location of the intersection on the topmost scan line; dx, the amount by which the x value changes from scan line to scan line; and dy, the number of intersections that the edge will generate.
The scan line conversion method generates the image from top to bottom, one scan line at a time. As it does so, it maintains an active edge list, sorted by x, listing all edges that cross the current scan line 110. Successive pairs of edges demarcate portions of the scan line in which the object is visible and are used to generate the display for the current scan line 110. After displaying a current scan line 110, each edge record is updated for the next scan line (x:=x+dx, dy:=dy-1). Occasionally these changes require edges in the active edge list to be interchanged so that the list remains sorted by x. Some edges also will terminate and be removed from the list, in addition to any new edge list entry 109 in the current scan line entry in list of edges 108 being incorporated into the current edge list 111.
As described in U.S. patent application No. Ser. No. 08/053,219 filed Apr. 28, 1993, claiming priority from Australian Provisional Patent Application No. PL 2150, of filed Apr. 29, 1992 entitled "Object Based Graphics Using Quadratic Polynomial Fragments", a method has been proposed for the representation of graphic objects by the use of quadratic polynomial fragments. Furthermore, prior to scan-line conversion, it is preferably to arrange the objects in such a manner that facilitates rendering by scan-line conversion.